1. Field of the Invention
The present invention relates to a manganese oxide thin film and an oxide laminate. More specifically, the present invention relates to a manganese oxide thin film and an oxide laminate undergoing Mott transition by controlling temperature, electric field, magnetic field or exposure to light and undergoing switching of electrical, magnetic or optical properties thereof.
2. Background of the Related Art
It is a recent concern that the scaling law, that provides guidance for performance improvement of semiconductor devices, is finally imposing an end. With this concern, materials, other than the all-time used silicon, have attracted attention as allowing novel operating principles. For example, in the field of spintronics incorporating the spin degree of freedom, development is underway for realizing high-density non-volatile memories which allow high speed operation as fast as DRAM (dynamic random access memories).
Meanwhile, studies on strongly-correlated electron system materials have also been advanced to which the band theory, the fundamental theory of semiconductor device design, cannot be applied. During the studies, materials have been found which undergo huge and rapid changes in physical properties resulting from phase transition of electron systems. In strongly-correlated electron system materials, not only the spin degree of freedom but also the orbital degree of freedom is involved in the state of a phase of the electron systems, and thus various electronic phases emerge with different ordering formed by spin, electron and orbital. A typical example of a strongly-correlated electron system material is perovskite-type manganese oxides which are known to have electron systems exhibiting a charge-ordered phase in which 3d electrons of manganese (Mn) are in order due to a first-order phase transition or an orbital-ordered phase in which orbitals are in order.
In the charge-ordered phase and orbital-ordered phase, the electric resistance is high due to carrier localization and thus the electronic phase is insulating. The electronic phase has a magnetic property which corresponds to an antiferromagnetic phase due to superexchange and double exchange interactions. There are many cases, however, in which the electronic states of charge-ordered phases or orbital-ordered phases should be regarded as semiconducting, because although carriers in the charge-ordered phases and orbital-ordered phases are localized, the resistance is lower than that of so-called band insulators. However, the electronic phase of charge-ordered phases and orbital-ordered phases are herein referred to as an insulator phase as is customary. To the contrary, the electronic phase having a low resistance and a metal-like behavior exhibits a ferromagnetic phase due to aligned spins. Although there are various definitions for the metallic phase, the metallic phase herein refers to “the phase having a positive temperature differential coefficient of resistivity”. Correspondingly, the insulator phase can be re-defined as “the phase having a negative temperature differential coefficient of resistivity”.
It is disclosed that the phenomena in which various switching capabilities emerge are observed in single-crystal bulk materials having any of electronic phases among the charge-ordered phase, the orbital-ordered phase and a phase in which both charge and orbital are ordered, i.e., a charge and orbital ordered phase, (see Japanese Patent Application Laid-open Nos. H8-133894, H10-255481, and H10-261291 (respectively, Patent Documents 1 to 3). These phenomena are typically observed as a huge change in resistance or transition between an antiferromagnetic phase and a ferromagnetic phase. For example, a change in resistance by orders of magnitude in response to a magnetic field application is well known as the colossal magnetoresistive effect.
In order to prepare practical devices such as electronic devices, magnetic devices, as well as optical devices, which utilize these phenomena for exhibiting switching capabilities, it is required to effectuate the phenomena causing the switching capabilities in a temperature range at or above room temperature (e.g., 300 K or above). However, the switching capabilities disclosed in Patent Documents 1 to 3 are all confirmed at a low temperature such as at or below the liquid nitrogen temperature (77 K). The perovskite-type manganese oxide disclosed in these Patent Documents is a laminate in which, provided that the chemical composition is designated as ABO3, atomic layers are repeatedly stacked such as an AO layer, a BO2 layer, an AO layer and so on. The crystal structure of such a laminate is herein represented as AO—BO2-AO. In the perovskite unit cell, an A site, a B site and an O (oxygen) respectively occupy the vertex, the body centre and the face centre. Manganese is located at the B site.
In Patent Documents 1 to 3, the type of the element or ion which occupies the A sites of the perovskite crystal structure is considered to be involved in a decrease in the temperature at which the switching phenomenon is observed, i.e., at which the charge-orbital ordering appear in the perovskite-type manganese oxide (hereinafter referred to as “appearance temperature”). Simply stated, the appearance temperature is decreased because of the random occupation of the A sites of the perovskite crystal structure by cations of trivalent rare earth (hereinafter designated as “R”) and divalent alkaline earth (“Ae”). To the contrary, it is also known that the transition temperature to the charge-ordered phase can be raised to about 500 K if the elements or ions at the A sites are ordered to be AeO—BO2—RO—BO2-AeO—BO2—RO—BO2— . . . . Hereinafter “A site ordering” refers to regular arrangement of ions at A sites such as those disclosed herein and “A-site ordered perovskite-type manganese oxide” refers to the perovskite-type manganese oxide with A site ordering. A group of materials which exhibits such a high transition temperature is characterized in that the materials contain Ba (barium) as an alkaline earth (Ae). It has been reported that, for example, the oxides containing Ba as an alkaline earth (Ae) and Y (yttrium), Ho (holmium), Dy (dysprosium), Tb (terbium), Gd (gadolinium), Eu (europium) or Sm (samarium), which have lower ionic radii, as a rare earth element (R) have a transition temperature above room temperature.
In order to effectuate devices such as electronic devices, e.g., magnetic devices, as well as optical devices, which utilize these phenomena, it is required to prepare the perovskite-type manganese oxide in a thin film form and effectuate the switching phenomenon. However, there has been a problem such that the switching capabilities are difficult to be obtained when the thin film is formed on a (100)-oriented substrate, because the lattice deformation, referred to as Jahn-Teller mode which is required for phase transition to the charge-ordered phase or the orbital-ordered phase is suppressed due to the in-plane 4-fold symmetry.
On the other hand, Japanese Patent Application Laid-open No. 2005-213078 (Patent Document 4) discloses the formation of a perovskite oxide thin film utilizing a (110)-oriented substrate. According to the disclosure of Patent Document 4, when the in-plane 4-fold symmetry is broken in the (110)-oriented substrate, shear deformation of the crystal lattice is permitted upon switching of the formed thin film. When shear deformation occurs, the crystal lattices are oriented parallel to the substrate plane while the charge-ordered plane or the orbital-ordered plane is non-parallel to the plane of substrate surface.
Japanese Patent Application Laid-open No. 2008-156188 (Patent Document 5) also discloses an example of the A-site ordered perovskite-type manganese oxide which is in the form of a thin film. This disclosure reports an application and light irradiation method in which, once an amorphous thin film is deposited, laser annealing is carried out for crystallization and A-site ordering. It is actually confirmed by electron diffraction that A-sites are in order in a SmBaMn2O6 thin film formed on a (100)-oriented SrTiO3 substrate (lattice constant: 0.3905 nm).
However, the A-site ordered perovskite-type manganese oxide has such a problem that the degree of A site ions order significantly affects the temperature at which the switching phenomenon is effectuated, i.e., the appearance temperature of charge-orbital ordering. Particularly in the case of a thin film of the A-site ordered perovskite-type manganese oxide, the degree of A site ion order may be reduced even with an introduction of defects in the formed thin film or a slight departure in the composition of the thin film. Moreover, the thin film on the (110)-oriented substrate reported in Patent Document 4 has such a problem that it does not contribute to any of reduction in the degree of order and reduction in the appearance temperature. Thus conventional perovskite-type oxide thin films have unsolved problems regarding the low appearance temperature of charge-orbital ordering and the appearance temperature of charge-orbital ordering that is at or above room temperature depending on the degree of A site order and the resulting instability.